1. Field of the Invention
This invention relates generally to spectroscopy and, more particularly, to an improved Fourier transform infrared spectrometer utilizing a detector having an infrared active area that can be controllably varied in size to optimize performance of the spectrometer for any of a variety of applications.
2. Discussion of the Prior Art
Fourier transform infrared (FT-IR) spectrometers are widely used in the analysis of chemical compounds. By measuring the absorption of infrared radiation by an unknown sample at various wavelengths in the infrared spectrum, and comparing the results with known standards, these instruments permit considerable useful information with respect to the chemical make-up of the unknown sample to be obtained. For an excellent discussion of these instruments, their theory of operation, design, and various applications, reference is made to the book titled Chemical Infrared Fourier Transform Spectroscopy, by Peter R. Griffiths, published by John Wiley & Sons (1975).
In a typical FT-IR spectrometer, infrared radiation from an infrared emitting source is collected, passed through an interferometer, passed through the sample to be analyzed, and finally brought to focus on an infrared detector. The interferometer system, in combination with the sample, modulates the intensity of the infrared radiation that impinges on the detector, and thereby forms a time variant intensity signal. It is the function of the detector to convert this time variant intensity signal to a corresponding time varying current. The current, in turn, is converted to a time varying voltage, which is presented to an analog-to-digital converter, and then stored as a sequence of digital numbers to be processed in a processor associated with the spectrometer.
Since the amount of infrared energy that impinges on the detector in an FT-IR spectrometer is usually very small, the detector must have a relatively high sensitivity and signal-to-noise ratio. While room temperature thermal detectors may be used for some spectrometer applications, many other applications require a much greater sensitivity than that which can be provided by room temperature detectors. In such higher sensitivity applications, cryogenically cooled semiconductor photon detectors, such as those fabricated from mercury cadmium telluride (MCT), are commonly used. MCT detectors are typically cooled with liquid nitrogen to a temperature of about 77 degrees Kelvin. This is accomplished by mounting the MCT detector on the end of a "cold finger" positioned within a vacuum-sealed dewar. The cold finger is in thermal contact with a liquid nitrogen reservoir. The infrared radiation to be detected passes to the MCT detector through an infrared transmissive window in the dewar housing. Since MCT provides excellent response to infrared radiation when cooled to the liquid nitrogen temperature, the sensitivity of such detectors is greatly improved when compared with room temperature detectors.
MCT detectors follow traditional detection sensitivity theory. More specifically, the MCT detector contributes noise to its output signal in direct proportion to the square root of the active area of the detector. At the same time, the detector converts incident infrared intensity signals into currents in direct proportion to the active area of the detector. Thus, the sensitivity of the detector, as measured by the signal-to-noise ratio of its output signal, increases in proportion to the square root of the detector active area. This sensitivity is an important parameter to the performance of general purpose FT-IR spectrometers. Its importance is best illustrated by considering the following three experiments common to the application of such spectrometers.
A. Analysis of large, clear samples
In the case where the sample being analyzed in the spectrometer is relatively large and relatively clear, the intensity of the infrared signal that impinges on the MCT detector is relatively high, and has a tendency to saturate the detector. Saturation occurs when the intensity of the signals incident on the detector is so high that the signals overwhelm the ability of the detector to convert photons into elements of current. Saturation, in turn, produces nonlinearity in the output of the detector, which degrades the accuracy of the spectrometer measurements. Thus, in the case of large, clear samples, an MCT detector with a relatively small active area is more desirable than a similar detector with a larger active area.
B. Analysis of large, opaque samples
In the case where the sample being analyzed is relatively large but nearly opaque, the intensity of the infrared signal that impinges of the MCT detector is very low, requiring maximum sensitivity from the detector. However, in this case, the physical size of the image of the infrared signal that is projected onto the detector is relatively large, typically on the order of three millimeters in diameter. The projected image remains this size and cannot generally be reduced at the detector focal plane due to practical limitations on the focusing ability of the reflecting optics in the spectrometer. Thus, in the case of large, opaque samples, it is desirable to employ an MCT detector with a relatively large active area, in order to maximize the infrared energy collected by the detector and the resulting signal-to-noise ratio of its output signal.
C. Analysis of small samples
In the case where the sample being analyzed is relatively small, the physical size of the image of the infrared signal that is projected onto the detector is also relatively small, typically on the order of one millimeter or less in diameter. The small size of the projected image results directly from geometrical and optical considerations involved in imaging the small sample from the focal plane of the sample to the focal plane of the detector. In this case, optimum sensitivity is achieved by matching the active area of the MCT detector to the physical size of the spectrometer's projected image. If the detector has an active area that is larger than the projected image, sensitivity decreases, since the outer portions of the detector, while contributing noise, are not being utilized to collect infrared energy.
Accordingly, it can be seen that, depending upon the particular application of the FT-IR spectrometer, the nature of the sample being analyzed, and other such factors, optimal performance is obtained by use of either a small or a large area MCT detector. However, these detectors, together with their associated dewar, cooler, optical and electronic components, are quite expensive, very often one of the most expensive parts of the entire spectrometer system. They are also rather difficult to position precisely at the proper focus of the spectrometer. Moreover, proper utilization of either a small or a large detector in an FT-IR spectrometer involves several other changes to the optics and electronics of the spectrometer, which makes the substitution of one detector for another rather difficult and time consuming. These overall considerations have typically prompted spectrometer vendors to supply only one or the other of these detectors (large or small area, but not both) with their instruments, depending upon the end user's particular application. Those end users desiring to use a single spectrometer for a variety of different applications have been compelled either to settle for less than optimal performance in some applications, or to bear the added expense and inconvenience of possessing and substituting multiple detectors.
There thus exists a need in the art for a spectrometer which can provide optimal performance in any of a variety of applications without the requirement for more than one detector.